At the present time, the only available means for readily assessing fish quality is the human sense of smell. Government agencies charged with assuring high standards of quality for fish intended for human consumption employ specially-trained and highly-experienced persons for this purpose. By the time fish reaches the home or supermarket, however, significant additional deterioration of the fish may have taken place. Also, only a relatively small percentage of the total fish for consumption can be governmentally-inspected. For the average individual, the human sense of smell is an imprecise and unreliable indicator of fish quality because by the time the fish develops an odor sufficiently objectionable to the average person, the fish has usually deteriorated well beyond the point of palatability. Thus, there is a need for a means of assessing fish quality which is quick, easily used by the average individual, free of expensive or complex equipment and, most important, highly accurate.
Much work has been done on various chemical methods for determining the quality of fish. For example, U.S. Pat. No. 2,626,855 for a "Seafood Spoilage Indicating System" employs a wooden stick impregnated with a chemical indicator which, by changing color, indicates the presence of acid conditions normally indicating putrefaction. This patent, however, is only capable of determining when "spoilage" has occurred, that is, when the seafood has so far deteriorated that it is no longer safe to consume. It is not-possible by the method of this patent to determine the "freshness" of the seafood short of spoilage conditions. For example, while it may be safe to consume fish which has been dead for several days but is not yet spoiled, "fresh" fish is more palatable and generally more tasty. Similarly, U.S. Pat. No. 2,485,566 for a "Method and Device for Indicating Spoilage" is not suitable for determining the quality of fish apart from spoilage conditions.
Efforts have also been made to determine the "freshness" of fish. One such method has focused on measuring the concentration of hypoxanthine in fish. Hypoxanthine is formed in dead fish by the breakdown of adenosine triphosphate (ATP), a natural biological substance found in live fish. Initially there is a build up in the level of hypoxanthine after the fish is harvested, but as bacteria begin to multiply and consume the hypoxanthine, the level drops off. Therefore, hypoxanthine levels in fish are indicative of freshness in terms of how long the fish has been dead as well as spoilage in terms of the multiplication of bacteria. Attempts to use hypoxanthine levels in assessing fish quality are reported in the following publications: L.R. Beuchat, "Hypoxanthine Measurement in Assessing Freshness of Chilled Channel Catfish," J. Agr. Food Chem., Vol. 21, p. 453 (1973); L.C. Dugal, "Hypoxanthine In Iced Freshwater Fish," J. Food Res. Bd. Can., Vol. 24, p. 2229 (1967); J. Spinelli, M. Eklund and D. Miyauchi, "Measurement of Hypoxanthine In Fish as a Method of Assessing Freshness," J. Food Sci., Vol. 29, p. 710 (1964); and T. Saito, K. Arai and M. Matsuyoshi, "A New Method for Estimating The Freshness of Fish," Bull. Japan Sci. Fish., Vol. 23, p. 265 (1959).
Each of these publications, however, discuss methods requiring sophisticated equipment or controls normally found only in a laboratory. A somewhat less complicated method for determining the levels of compounds in biological fluids is described in U.S. Pat. No. 3,099,605. Filter paper strips are impregnated with a solution comprising two enzymes, an indicator whose color is affected by hydrogen peroxide in the presence of one of the enzymes, a buffer and a stabilizer. For example, in testing for hypoxanthine, the patent suggests using a solution of xanthine oxidase, orthotolidine dihydrochloride, peroxidase and a buffer. The solution is said to turn blue in the presence of xanthine or hypoxanthine. The principal difficulty with this method is that it requires the simultaneous use of two enzymes, one to form hydrogen peroxide and the second to activate the dye system for making measurements. A two-enzyme system of this type is more difficult to prepare and control and is not as stable in storage as a single enzyme system.